Lectins are defined as proteins which specifically bind carbohydrates of various types. Initial interest was focused on those isolated from plants such as concanavalin A and ricin agglutinin. These lectins, it was found, were useful in protein purification procedures due to the glycosylation state of a number of proteins of interest. Among the soluble lectins, there appear to be a number of varieties with varying molecular weights and/or carbohydrate specificities. Sparrow, C. P., et al., J. Biol. Chem. (1987) 252:7383-7390 describe three classes of soluble lectins from human lung, one of 14 kD, one of 22 kD, and a third of 29 kD. All of these lectins are specific to .beta.-D-galactosides. The carbohydrate specificities of the 14 kD class are for the most part similar, but the larger molecular weight species tend to have different specificities. Other species are also noted as showing more than one soluble .beta.-D-galactoside-binding lectin, including mouse (Roff, C. F., et al., J. Biol. Chem. (1983) 258:10637-10663); rat (Cerra, R. F., et al., J. Biol. Chem. (1985) 260:10474-10477) and chickens (Beyer, E. C., et al., J. Biol. Chem. (1980) 255:4236-4239). Among the various .beta.-D-galactoside-specific soluble lectins, ligand specificity is considerably different, and the approximately 14 kD group appears distinct from the 22 kD and 29 kD representatives described by Sparrow, et al., supra.
Recently, however, interest has focused on a group of lactose-extractable lectins which bind specifically to certain .beta.-D-galactoside containing moieties and are found in a wide range of mammalian, in-vertebrate, avian, and even microbial sources. All of the lectins in this class appear to contain subunits with molecular weights of about 12-18 kD. Furthermore, these lectins can be readily classified by virtue of a simple diagnostic test: their ability to agglutinate trypsin-treated rabbit red blood cells is specifically inhibited by certain .beta.-D-galactose-containing moieties. Thus, although the lectins themselves agglutinate trypsinized rabbit erythrocytes, the agglutination can be inhibited by, for example, lactose, thiodigalactoside and certain other .beta.-D-galactose containing moieties. Other common characteristics include no requirement for metal ions in effecting agglutination and the required presence of a reducing agent such as a thiol.
Gitt, M. A. et al., Proc. Natl. Acad. Sci. U.S.A. (1986) 83:7603-7607 obtained two cDNA clones from immunoscreening a human hepatoma cDNA library with an antiserum specific to a human lung lectin. Gitt et al. partially sequenced the cDNAs and the lectins. Gitt compared these sequences with that of the human lung chicken lectin. Although there were marked similarities with chicken and lung lectin, Gitt et al. concluded "In contrast with lung [encoding one form of HL-14 lectin], human hepatoma appears to express two other forms of HL-14" (page 7607). Kasai, K. et al., in Japanese Kokai 60/184020 describe a human placental lectin of approximately 14 kD. The sequence of this placental lectin was shown by the same group to be somewhat similar to that isolated from chick tissues (Ohyama, Y., et al., Biochem. Biophys. Res. Commun. (1986) 134:51-56). The chick-derived lectin was shown to be similar in structure to that of discoidin I, which is a lectin also observed during certain developmental stages of the cellular slime mold Dictyostelium discoideum.
Caron, M., et al., Biochim. Biophys. Acta (1987) 925:290-296 describe the purification and characterization of lectins from rat and bovine brain tissue. deCabutti, N. E. F., et al., FEBS Letters (1987) 223:330-334 describe a lectin from amphibian ovary. The isolation from eel of a similar "electrolectin" had previously been described by Levi, G., et al., J. Biol. Chem. (1981) 256:5735-5740. An additional analogous 14 kD lectin was produced by cloning and expression of cDNA derived from various murine fibrosarcoma cell lines by Raz, A., et al., Experimental Cell Research (1987) 173:109-116. A rat lung 14 kD lectin, and the cDNA encoding it were described by Clerch, L. B., et al., Biochemistry (1988) 27:692-699. Joubert, R., et al., Develop. Brain Res. (1987) 36:146-150 describe the isolation of lectins from rat brain which are capable of agglutinating brain cells. Raz, A., et al., Cancer Res. (1981) 41:3642-3647 describe a variety of lectins from neoplastic cells of various mammalian species.
Paroutaud, P., et al., (Proc. Natl. Acad. Sci. U.S.A. (1987) 84:6345-6348) compared the amino acid sequences of several animal lectins including those from chick, eel, human placenta, human lung, and two hepatoma-derived lectins (all of these lectins described as referenced above). Only the chicken lectin contains an "N-linked" glycosylation site, which is not conjugated to saccharide. No known mammalian lectin in this family has an N-linked glycosylation site.
Although several of the above references disclose some structural similarities with the present invention, none of the references teach the same bioactivity of the unique lectin of the present invention.
The preferred lectins of the present invention are isolated from the human promyelocytic leukemia cell line HL-60 or human placenta tissue. Lectins have been isolated from the HL-60 cell line by others, but they are markedly different from the lectins of the present invention. Paietta, E., et al., Cancer Res. (1988) 48:280-287 describe a membrane-bound (not soluble), 17 kd lectin which recognizes N-acetyl neuraminic acid as well as galactose terminating biantennary oligosaccharide structures. Unlike other 14 kd lectins, this 17 kd lectin is not inhibited by complex galactose saccharides such as thiodigalactoside and does not require reducing thiol groups for binding activity.
Thus, ligand specificity and biodistribution of the lectin protein described herein are an abrupt departure from the earlier disclosed lectins.
Because the activities of lectins in regulating the immune system and mediating other forms of intercellular communication are so subtle in nature and so critically tuned to the host environment, subtle changes in structure can result in a wide range of such regulators with differing therapeutic and diagnostic uses. As described above, a number of members of the class of .beta.-D-galactose-binding soluble lectins weighing approximately 14 kD are known in the art. However, while these lectins have some similarities, they are not interchangeable therapeutically or diagnostically. In addition, it appears that for lectins which can be glycosylated, the extent and nature of the glycosylation can be manipulated to alter important lectin properties (e.g., circulating half-life, metabolism in vivo, solubility, stability, and specific activity).
Levy et al. (Eur. J. Immunol. (1983) 13:500-507) reported that electrolectin binds to peripheral blood and lymph node lymphocytes and is mitogenic. When Levy et al. administered electrolectin to rabbits simultaneously with acetylcholine receptor, it prevented the development of a myasthenia gravis-like condition. Administering electrolectin after development of myasthenia gravis caused complete recovery, in spite of high antibody levels specific for the acetylcholine receptor. Because electrolectin did not interfere with acetylcholine interaction with its receptor, Levy et al. proposed that electrolectin had an effect on the immune system.
Prominent diseases in which there is an immune system dysfunction include autoimmune diseases such as myasthenia gravis (MG), rheumatoid arthritis (RA) systemic lupus erythematosus (SLE), multiple sclerosis (MS) and juvenile arthritis. Typically MG, RA, SLE and MS are treated first with corticosteroids. Steroidal drugs have been used for decades and their adverse effects are well known. Adverse effects that can be anticipated in all patients on prolonged steroid therapy include osteoporosis, truncal obesity, impaired wound healing, infections and growth arrest in children. Less frequently occurring adverse effects include myopathy, hypertension, hyperlipidemia, diabetes mellitus and cataracts. Severe side effects may develop and require patient monitoring. These include glaucoma, intracranial hypertension, intestinal perforation, and ulcers.
If MG, RA, SLE or MS become refractory to steroids, then increasingly toxic drugs are employed, including azathioprine, methotrexate and cyclophosphamide. The primary effect of azathioprine is inhibiting DNA synthesis, thus lowering numbers of T and B lymphocytes. In addition, azathioprine inhibits the mixed lymphocyte reaction and immunoglobulin production, but does not consistently affect delayed-type hypersensitivity. The major adverse effect of azathioprine is pancytopenia, particularly lymphopenia and granulocytopenia. Consequently, there are increased risks of viral, fungal, mycobacterial and protozoal infections. An increased rate of lymphoreticular malignancies has been reported in kidney transplant patients, but not in patients with RA.
Methotrexate inhibits folic acid synthesis and is cytotoxic, suppressing bone marrow. At the low doses used for RA, methotrexate should not decrease the numbers of lymphocytes; but IgM and IgG are reduced. Side effects include pneumonia, nausea, stomach upsets, mouth ulcers, leukopenia, thrombocytopenia, and a form of hepatic fibrosis, which can only be diagnosed by liver biopsy.
Cyclophosphamide is also used in RA therapy. It is metabolized in the liver to a compound which cross-links DNA. Cyclophosphamide is cytotoxic, with severe toxicity seen even at low doses. It affects RA by reducing numbers of B- and T-lymphocytes, decreasing the immunoglobulin concentrations and diminishing B-cell responsiveness to mitogenic stimuli. Hair loss, infections, and powerful nausea are common. With prolonged administration, patients develop malignancies at an increased rate.
Cyclosporin does not suppress white cells, but it is a powerful immunomodulatory drug and is effective in treating rheumatoid arthritis. However, an important side effect is renal toxicity.
Monoclonal antibodies to CD4 have been used in autoimmune diseases, but they cause nonspecific immunosuppression. It has been recommended that new therapies interfere with the initial presentation of specific inciting antigens to T-lymphocytes. (Wraith et al., Cell (1989) 57:709-715).
Other drugs have been used specifically in RA, including gold salts, antimalarials, sulfasalazine and penicillamine. Gold salts are given intramuscularly and their effect may not be seen for months. Adverse effects of gold treatment include bone marrow aplasia, glomerulonephritis, pulmonary toxicity, vasomotor reactions and inflammatory flare. Antimalarials exert several effects on the immune system without decreasing the numbers of lymphocytes. The most serious side effects of antimalarials include retinal pigment deposition, rash and gastrointestinal upset. Sulfasalazine has several effects which contribute to its effect on RA; however, it has numerous side effects. Penicillamine has been successfully used in RA; however, its numerous side effects have limited its use. Penicillamine has been reported to cause other autoimmune diseases, including myasthenia gravis and SLE.
When patients receive allografts (transplanted tissue from other humans or other sources), their immune systems can destroy the allografts in short order were it not for the administration of immunosuppressant drugs. A number of different organs and tissues are now transplanted, including the kidneys, heart, lungs, skin, bone marrow, cornea, and liver. Drugs frequently used in transplant patients include cyclosporin, azathioprine, rapamycin, other macrolides such as FK506, prednisone, methylprednisolone, CD4 antibodies and cyclophosphamide. Frequently these drugs must be given in higher doses and for longer periods to transplant patients than to patients with autoimmune diseases. Hence, side effects from these drugs (discussed above) may be more common and severe in transplant patients.
What was needed before the present invention is a drug that would selectively treat autoimmune diseases and transplant rejection without the severe side effects of the previously known therapies.